Apparatus and method for air drying with reduced oxygen enrichment

ABSTRACT

Adsorbers used to dry air frequently contain 4A or 13X molecular sieve zeolites, which are intermittently reactivated by thermal regeneration. When the air pressure in a thermally regenerated adsorber is increased, such as during start-up, after adsorbent reloading, after apparatus repair, maintenance or inspection, or after repressurization following thermal regeneration, nitrogen adsorbs preferentially relative to oxygen in activated 4A and 13X molecular sieve zeolites. This produces an oxygen-enriched, high pressure gas within the adsorber vessel which emerges immediately after the adsorber is brought on line. The oxygen-enriched, high pressure gas can cause fires, explosions, and other deleterious effects in and downstream of such adsorber vessels. This invention provides thermally regenerated air driers and air drying processes using adsorbent compositions that do not adsorb nitrogen preferentially to oxygen. Thermally regenerated air driers and drying processes using such adsorbent compositions prevent the problems associated with transient oxygen enrichment.

BACKGROUND OF THE INVENTION

Ambient air contains water vapor. Many industrial processes require asource of air with low water vapor content. Important reasons for theremoval of water vapor from air are to control the humidity ofmanufacturing atmospheres, to protect electrical equipment againstcorrosion, short circuits, and electrostatic discharges, to meetrequirements for chemical processes where moisture present in airadversely affects the process, and to prevent water adsorption inpneumatic conveying. Dried air is commonly produced at the point of use.Many types of air driers and air drying processes are used.

For many years, air driers have used adsorbent compositions, sometimescalled desiccants or simply adsorbents, to remove water from airstreams. For economic reasons, the adsorbent compositions used in airdriers are usually used more than once. Adsorbent compositions suitablefor use in air driers must be capable of adsorbing and desorbing water.Many different adsorbent compositions have been used in air drying.Adsorbent compositions used in air drying processes have containedcertain types of molecular sieve zeolites, silica gels, and activatedaluminas. Types 4A and 13X molecular sieve zeolites have been used inadsorbent compositions for air driers. Those molecular sieve zeoliteshave suitable water adsorbing and desorbing characteristics for use inair driers. These 4A or 13X molecular sieve zeolites are sometimes usedin combination with other adsorbents to remove water and sometimescarbon dioxide and hydrocarbons from air streams.

Air driers are generally designed with at least two adsorbent chambersor vessels. This arrangement permits production of a continuous supplyof dried air. Adsorbent chambers or vessels at least partially filledwith adsorbents are sometimes called "adsorbers". Typically, one chamberhaving "dry" adsorbent is connected with the air stream and water vaporin the air is adsorbed. While that chamber is adsorbing water, the otherchamber with "wet" adsorbent is isolated from the air stream and wateris removed from the adsorbent. The air drying operation is sometimescalled "adsorption" or simply "drying". The adsorbent drying operationis sometimes called "regeneration" or "reactivation". Air driers aredesigned to cycle between the chambers so that one operates in thedrying mode, while another operates in the regeneration mode.

A variety of regeneration processes are used. A common method is called"thermal" or "heat" regeneration. Thermal regeneration involves heatingthe adsorbent composition to a temperature at which its adsorptivecapacity is reduced. At the lower equilibrium adsorption capacity, thewater leaves the adsorbent surface and is removed by a stream of "purge"gas or by vacuum. The temperature to which the adsorbent compositionmust be heated is determined primarily by the degree to which the airmust be dried and the required rate of regeneration. Other factors beingequal, the dew point produced by a thermally regenerated air drier usingmolecular sieve zeolite adsorbent compositions will be lower as theregeneration temperature increases from 250° F. to 600° F. Regenerationtemperatures of 300° F. to 500° F. are usually employed. Thermalregeneration is commonly conducted at pressures below the pressures atwhich the air drying operation is conducted. Thermal regeneration mayalso be conducted at pressures about the same as the pressures at whichthe air drying operation is conducted.

Ambient air is typically provided to an adsorber as a pressurized airstream. Air drying is conducted at the pressures of the air stream beingdried. The air stream typically enters the adsorber for drying atpressures of 30 to several thousand pounds per square inch ("psia"),although some driers operate at pressures only marginally aboveatmospheric. The pressure in a chamber is typically reduced for thermalregeneration. Although the pressure is generally reduced for thermalregeneration, the external heat supplied to the adsorbent compositionprovides the primary thermodynamic driving force to regenerate theadsorbent composition in thermally regenerated air driers. For thatreason, thermal regeneration may also be carried out without asubstantial pressure reduction in the chamber.

After an adsorbent composition has been thermally regenerated, a coolingperiod may be used to reduce the adsorbent temperature to nearly that ofthe stream being processed before the vessel is connected to the streamfor drying. Cooling is usually regulated so as to leave the adsorbentcomposition at a temperature within about 50° F. of the air stream to bedried. If the pressure in the chamber was reduced for regeneration, thecooling is typically accomplished before the pressure in the chamber isincreased to the pressure of the air stream to be dried.

In sum, thermally regenerated adsorbent chambers or vessels typicallyoperate in a cycle which includes the following steps: adsorb,depressure, heat adsorbent, cool adsorbent and repressure. Lesscommonly, this operating cycle does not include the depressure andrepressure steps of regeneration. Regeneration is not the only occasionfor air pressure to be increased in an adsorbent chamber of a thermallyregenerated air drier containing activated or "dry" adsorbent. The useof any thermally regenerated air drier typically includes other eventsthat involve air pressure increases in adsorbent chambers containingactivated or "dry" adsorbent. Activated or "dry" adsorbent may be loadedinto a chamber at atmospheric pressure for a variety of reasons--duringthe start-up of a drier, or during the relacement of worn out or spentadsorbent. The pressure in the chamber increase to the pressure of theair stream to be dried, when the drier is put to use in the normaloperating cycle. The air pressure in adsorbent chambers may also bereduced below normal line pressure when a drier is "shut-down," becausedried air is not needed, or the drier is being repaired, worked on,inspected, or tested. When such a drier resumes normal use, the pressurein the chambers will increase to the pressure of the air stream to bedried. For these and other reasons, adsorbent chambers or vessels ofthermally regenerated air driers containing "dry" or activated adsorbentwill experience at least one air pressure increase step, and possiblymany air pressure increase steps, even in those driers that do notemploy pressure reduction as part of adsorbent regeneration. Thechambers of thermally regenerated air driers are generally sized topermit operation in the drying mode for periods of between about 1 to 24hours and more typically between about 2 to 8 hours.

Certain molecular sieve zeolites are used to produce oxygen enriched airby a pressure-swing adsorption-desorption process, sometimes called"heatless" adsorption-desorption. This process operates by selectiveadsorption of nitrogen from air by molecular sieve zeolites at highpressure and desorption at lower pressure. Molecular sieve zeolites,such as type 5A, that exhibit a particularly strong selectivity foradsorbing nitrogen in preference to oxygen are used in this process.This process is described generally in C. W. Skarstrom, U.S. Pat. No.2,944,627. The adsorbent chambers of a heatless adsorption-desorptionprocess are not regenerated by the use of external heat. A thermalregeneration step is not involved. Very short cycle times are used inheatless adsorption-desorption processes. The chambers of a heatlessadsorption-desorption process are typically sized to permit operation inthe adsorption mode for periods between about 10 seconds to 3 minutes.

BRIEF DESCRIPTION OF THE INVENTION

Fires and explosions sometimes occur after a thermally regeneratedadsorbent chamber is brought on line. In one instance, a thermallyregenerated air drier containing molecular sieve zeolite was placedbetween stages of an oil lubricated compressor. Detonation occurred atthe compressor stage immediately downstream of the thermally regeneratedair drier. After the compressor stage was rebuilt, another detonationoccurred in the same stage.

It is common commercial practice to use an adsorbent compositioncontaining substantial amounts of 4A or 13X molecular sieve zeolites inthermally regenerated air driers. When an adsorber of a thermallyregenerated air drier using an adsorbent composition containingsubstantial amounts of 4A or 13X molecular sieve zeolites is repressuredfollowing thermal regeneration, oxygen-enriched air occupies theinterstitial space in the adsorber and emerges from the adsorber whenconnected to the air stream to begin the adsorption step. This transientoxygen enrichment also occurs whenever the pressure in such an adsorberis increased from a relatively low pressure, such as atmosphericpressure, to the pressure of the air stream to be dried, such as occursin connection with adsorbent loading or reloading, or with drier shutdown and restarting for a variety of reasons. Prior to this invention,it was not recognized that transient oxygen enrichment may occur in andbe transported downstream of thermally regenerated air driers. Prior tothis invention, it was not recognized such that transient oxygenenrichment contributes to the likelihood and the severity of fires andexplosions, and to other undesirable effects, such as acceleratedoxidation (e.g., rust in pipes, changes in biological specimens).Selective nitrogen adsorption and gas phase oxygen enrichment areheretofore unrecognized, unintended, and generally undesirable events inthermally regenerated air drying processes and driers, which containsubstantial amounts of molecular sieve zeolites such as types 4A and13X. The present invention prevents these effects in thermallyregenerated air driers and processes.

The thermally regenerated air driers, adsorbent chambers, and air dryingmethods of the invention reduce this undesirable oxygen enrichment byusing an adsorbent composition comprising one or more molecular sievezeolites capable of adsorbing water vapor from air and beingsubstantially free of molecular sieve zeolites capable of substantiallyselectively adsorbing nitrogen in preference to oxygen. One suchadsorbent composition comprises type 3A molecular sieve zeolite and issubstantially free of molecular sieve zeolites, such as types 4A and13X, that substantially selectively adsorb nitrogen in preference tooxygen. In contrast to an adsorbent composition consisting essentiallyof one or more of the commonly used 4A and 13X molecular sieve zeolites,neither oxygen nor nitrogen adsorbs significantly on an adsorbentcomposition consisting essentially of 3A molecular sieve zeolite becausethe micropores of 3A molecular sieve zeolite are too small. The criticaldiameters of nitrogen and oxygen are about 3.6Å and 3.5Å respectively,both of which are larger than the 3.3Å diameter of the micropores of 3Amolecular sieve zeolite (D. W. Breck, "Zeolite Molecular Sieves", Wiley,New York, 1974). Hence transient oxygen enrichment and its deleteriouseffects will not occur in and downstream of thermally regeneratedadsorbers which contain such an adsorbent composition.

This invention relates to thermally regenerated air drying equipment. Ingeneral, this invention provides an apparatus for drying air comprising(a) at least one adsorbent chamber, (b) such chamber containing anadsorbent composition comprising one or more molecular sieve zeolitescapable of adsorbing water vapor from air, wherein such composition issubstantially free of molecular sieve zeolites capable of substantiallyselectively adsorbing nitrogen in preference to oxygen, (c) means forconnecting such air stream to such chamber to adsorb water andincreasing the air pressure in such chamber at least once, (d) means forisolating such chamber from such air stream, and (e) means for heatingthe adsorbent composition in such isolated chamber to substantiallydesorb water from such adsorbent composition. The means (c) may includemeans for providing an air stream to the chamber. The means (e) may alsobe capable of reducing the air pressure in such isolated chamber priorto heating and increasing the air pressure in such chamber followingheating. The means (e) is also capable of substantially removingdesorbed water from such chamber. Preferably, such an apparatuscomprises at least two adsorbent chambers, each containing such anadsorbent composition, means for connecting an air stream to suchchambers to adsorb water, means for isolating such chambers from the airstream to desorb water, and means for controlling such connecting andisolating means so that the air stream is connected to one such chamberto adsorb water while another such chamber is isolated from such airstream to desorb water. Such control means permits the apparatus toproduce a continuous supply of dried air by alternating the air streambetween such chambers so that one such chamber adsorbs water from theair while water is being desorbed from another such chamber. Thisapparatus permits thermally regenerated air drying without substantiallyincreasing the oxygen concentration of the air in such isolated chamberas the air pressure in the chamber is increased after regeneration orany other operation involving such a pressure increase.

This invention also relates to thermally regenerated air dryingprocesses for reducing the oxygen enrichment problem. In general, thisinvention provides a method of air drying comprising (a) adsorbing watervapor from an air stream by contacting such air stream with an adsorbentcomposition comprising one or more molecular sieve zeolites capable ofadsorbing water vapor from air, wherein such composition issubstantially free of molecular sieve zeolites capable of substantiallyselectively adsorbing nitrogen in preference to oxygen, and isolatingsuch adsorbent composition from such air stream, (b) increasing thepressure of air in contact with such adsorbent composition prior to orduring at least one such adsorbing step (a), and (c) heating suchadsorbent composition to desorb water. This process also involvessubstantially removing desorbed water from contact with the adsorbentcomposition.

The apparatus and methods of the invention may use any adsorbentcomposition containing one or more molecular sieve zeolites to adsorbwater vapor from air and being substantially free of molecular sievezeolites capable of substantially selectively adsorbing nitrogen inpreference to oxygen.

DESCRIPTION OF THE INVENTION The Figures And Table

FIG. 1 shows a typical thermally regenerated air drier.

FIG. 2 illustrates predicted axial composition profiles afterpressurization of a thermally regenerated adsorber containing 13Xmolecular sieve zeolite. The adsorber was assumed to contain activatedor freshly regenerated 13X molecular sieve zeolite and air at 1.0 atm atthe start of an isothermal repressurization or pressurization step. Airat 273° K. was assumed to enter the adsorber during the isothermalrepressurization step until the total pressure in the adsorber reached5.0, 7.5, or 10.0 atm.

FIG. 3 illustrates predicted composition breakthrough curves for anadsorption step of a thermally regenerated adsorber containing 13Xmolecular sieve zeolite. Air at 273° K. is assumed to enter the adsorberduring an isothermal adsorption step, which immediately follows thecorresponding isothermal repressurization or pressurization step shownin FIG. 2. The dimensionless time equals the number of bed volumes whichare assumed to pass through the adsorber, based on an empty vessel.

FIG. 4 illustrates a predicted increase in the upper flammability limitof a hydrocarbon fuel relative to air in the effluent gas emerging froma thermally regenerated adsorber containing 13X molecular sieve zeolite.The dimensionless time equals the number of bed volumes which areassumed to have passed through the adsorber, based on an empty vessel.

Table 1 lists the minimum autoignition temperatures of certainhydrocarbon fuels in degrees Kelvin.

DESCRIPTION

This invention relates to thermally regenerated air drying equipment andair drying processes.

FIG. 1 illustrates one type of thermally regenerated air drier and airdrying process that may employ this invention. The ambient air stream tobe dried is pressurized by a compressor, 1, enters an inlet switchingvalve, 2, passes through the left adsorbent chamber to adsorb watervapor, 3, through an outlet switching valve, 4, to the dry air outlet.During this adsorption or drying step, the air pressure is typicallyabout 30 to several thousand psia. The temperature of the air stream istypically about 40° F. to 130° F. The flow rate may vary over a widerange depending on drying requirements. A portion of dried air passesthrough a purge throttling valve, 5, then passes through a heater, 6,and the outlet switching valve, 4. Heated purge gas then enters theright adsorbent chamber, 7, where it is dispersed through the wetadsorbent composition. Flow direction may be co-current orcounter-current to the drying flow. The pressure in the adsorbentchamber during this regeneration or reactivation step is typically about0.1 to 100 psig. The purge air, now carrying previously adsorbedmoisture, exits to the atmosphere through the inlet switching valve, 2,and the purge exhaust valve, 8. The regenerating adsorbent chamber isalso typically cooled to temperatures of about 40° F. to 150° F., forexample, by turning off or bypassing the heater, 6, prior to theadsorption step. At the end of the regeneration period, the purgeexhaust valve, 8, may close to repressurize the reactivated adsorbentchamber, 7. Switchover may then take place with both chambers at linepressure. Alternatively, the reactivated adsorbent chamber may berepressurized as the chamber is switched to connect it to the air streamfor air drying. The inlet and outlet switching valves reverse the airflows. The switching valves and purge exhaust valve may be controlledmanually or by conventional controllers adjusted to the requirements ofthe particular drier and process. Upon switchover, the left wetadsorbent chamber, 3, is depressurized, and its regeneration cycleinitiated, while the regenerated right chamber, 7, dries air at linepressure. Ports, manways or removable heads, 9, are typically a part ofeach adsorbent chamber for loading and unloading the adsorbentcomposition into the adsorbent chambers. A commercial thermallyregenerated air drier that is designed and operates in a manner capableof use in the invention is the Pall Trinity Micro Corporation's Type AHeat-Reactivated Dryer.

This is one example of a thermally regenerated air drier and air dryingprocess that may employ the invention. It will be apparent to thoseskilled in the art that there are other types of thermally regeneratedair driers and air drying processes that may employ the invention. Forexample, the regeneration step may be conducted at air pressures aboutthe same as those of the adsorption or drying step. This eliminates theneed for components and steps to depressure and repressure an adsorberduring each regeneration cycle. However, this does not eliminate theneed for components and a step to increase the pressure in an adsorbercontaining activated or "dry" adsorbent at least once prior to or duringan adsorption step. All thermally regenerated air driers and air dryingprocesses typically employ some mechanism and procedure for pressurizingthe adsorbent chambers, such as a pipe and/or a valve connected to areservoir of air at elevated pressure, or to a compressor, 1, or toboth. Such a thermally regenerated air drier and air drying processwould also typically have a device for depressurizing the adsorbentchambers, such as vent valve(s), 10. Depressurization may alternativelybe accomplished by the purge exhaust valve, 8, or other venting device.A pressure sensor and/or regulator, 11, might also be a part of such athermally regenerated air drier and air drying process, and may providealternate means for depressurizing and pressurizing the system. Such apressure indicator and/or regulator could be located downstream of,upstream of, or attached to the adsorbent chamber(s). Devices fordepressurizing and/or pressurizing are needed in a variety of instancesother than regeneration, including: (a) a start-up, including theinitial start-up, of a thermally regenerated air drier and air dryingprocess; (b) other start-up and shut-down instances as might occur, forexample, during intermittent operation, either planned or unplanned; (c)loading, unloading or replacing the adsorbent composition; (d) apparatusrepair, maintenance, inspection, troubleshooting, or testing; and (e)compensating for changes in the system pressure, such as a pressurechange due to a leak or a pressure change associated with a change inflow rate.

When an adsorber of such a thermally regenerated air drier using anadsorbent composition containing substantially 4A or 13X molecular sievezeolites is repressurized following thermal regeneration or the airpressure in such an adsorber is otherwise increased, oxygen-enriched airemerges from the adsorber during the beginning of the adsorption step.Transient oxygen enrichment increases the likelihood and the severity offires and explosions in and downstream of thermally regenerated airdriers that use an adsorbent composition capable of substantiallyselectively adsorbing nitrogen in preference to oxygen, such as acomposition containing substantially types 4A and 13X molecular sievezeolites.

A combination of fuel, oxidizer, and a source of ignition must bepresent to support combustion. Hydrocarbons and ignition sources arecommonly found in and downstream of thermally regenerated air driers.Compressors, pipelines with an oil film on the inside walls, and cokedeposits on adsorbent compositions are potential sources of hydrocarbonsand ignition. Compressors which could be downstream of thermallyregenerated air driers provide both necessary conditions for combustion.For example, autoignition and detonation of compressor lubricating oilsdue to an increased oxygen concentration in the compressor inlet gaseshave been reported in equipment used to produce oxygen and nitrogen fromair, commonly known as air separation facilities. Pipelines coated withoil on their interior and charged with air present a severe explosionhazard. Intermittent flow, especially a sudden start of air flow,facilitates the ignition of oil residues by depressing the autoignitiontemperature to about 425° K. The sudden start of flow when an adsorberis brought on line will coincide with the highest oxygen partialpressure of the adsorber effluent. Fires have been reported to occur inpipelines downstream of thermally regenerated air driers. However, priorto this invention, no one explained these incidents on the basis oftransient oxygen enrichment or disclosed how to reduce oxygen enrichmentby the choice of adsorbent composition.

The thermally regenerated air driers and air drying methods of theinvention reduce this undesirable oxygen enrichment by using anadsorbent composition containing molecular sieve zeolites to adsorbwater vapor while being substantially free of molecular sieve zeolitesthat substantially selectively adsorb nitrogen in preference to oxygen.One such preferred adsorbent composition consists essentially of type 3Amolecular sieve zeolite. In contrast to an adsorbent compositioncontaining substantial amounts of one or more of the commonly used 4Aand 13X molecular sieve zeolites, neither oxygen nor nitrogen adsorbssignificantly on an adsorbent composition consisting essentially of 3Amolecular sieve zeolite. Hence transient oxygen enrichment and itsdeleterious effects can not occur in and downstream of thermallyregenerated adsorbers which contain an adsorbent composition that doesnot substantially selectively adsorb nitrogen in preference to oxygen.

This invention provides an apparatus for drying air comprising (a) atleast one adsorbent chamber, (b) such chamber containing an adsorbentcomposition comprising one or more molecular sieve zeolites capable ofadsorbing water vapor from air, wherein such composition issubstantially free of molecular sieve zeolites capable of substantiallyselectively adsorbing nitrogen in preference to oxygen, (c) means forconnecting an air stream to such chamber to adsorb water and increasingthe air pressure in such chamber at least once, (d) means for isolatingsuch chamber from such air stream, and (e) means for heating theadsorbent composition in such isolated chamber to substantially desorbwater from such adsorbent composition. The means (c) may include meansfor providing an air stream to the chamber. The means (e) may also becapable of reducing the air pressure in such isolated chamber prior toheating and increasing the air pressure in such chamber followingheating. The means (e) is also capable of substantially removingdesorbed water from such chamber. Preferably, such an apparatuscomprises at least two adsorbent chambers, each containing such anadsorbent composition, means for connecting an air stream to suchchambers to adsorb water, means for isolating such chambers from the airstream to desorb water and means for controlling such connecting andisolating means so that the air stream is connected to one such chamberto adsorb water while another such chamber is isolated from such airstream to desorb water. Such control means permits the apparatus toproduce a continuous supply of dried air by alternating the air streambetween such chambers so that one such chamber adsorbs water from theair while water is being desorbed from another such chamber. Thisapparatus permits thermally regenerated air drying, withoutsubstantially increasing the oxygen concentration of the air in suchisolated chamber as the air pressure in the chamber is increased afterregeneration or any other operation involving such a pressure increase.

This invention provides a method of air drying comprising (a) adsorbingwater vapor from an air stream by contacting such air stream with anadsorbent composition comprising one or more molecular sieve zeolitescapable of adsorbing water vapor from air, wherein such composition issubstantially free of molecular sieve zeolites capable of substantiallyselectively adsorbing nitrogen in preference to oxygen, and isolatingsuch adsorbent composition from such air stream, (b) increasing thepressure of air in contact with such adsorbent composition prior to orduring at least one such adsorbing step (a), and (c) heating suchadsorbent composition to desorb water. This process also involvessubstantially removing desorbed water from contact with the adsorbentcomposition.

The apparatus and methods of the invention may use any adsorbentcomposition containing one or more molecular sieve zeolites capable ofadsorbing water vapor and being substantially free of molecular sievezeolites capable of substantially selectively adsorbing nitrogen inpreference to oxygen. An adsorbent composition is substantially free ofmolecular sieve zeolites capable of substantially selectively adsorbingnitrogen in preference to oxygen, if such composition does not causesubstantial transient oxygen enrichment of the air in an adsorbentchamber resulting from thermal regeneration and/or pressurization. Asuitable adsorbent composition may contain one particular type ofmolecular sieve zeolite adsorbent or may consist of a mixture ofdifferent types. Such adsorbent compositions may contain insubstantialamounts of types of adsorbents that do substantially selectively adsorbnitrogen in preference to oxygen, such as activated alumina or certainsilica gels, and, less preferably, such as types 4A and 13X molecularsieve zeolites. An insubstantial amount of such adsorbent is an amountthat is too small to cause any significant oxygen enrichment.

Adsorbent compositions may be used in the forms in which they arenormally used in thermally regenerated air drying applications.Adsorbent compositions may contain adsorbents and binders for adsorbentssuitable for use in thermal regenerated air drying. Molecular sievezeolite powders may be blended in the usual manner with inorganicbinders and formed into larger agglomerates of discrete size and shapesuitable for use in thermally regenerated air driers. The particle sizeof the zeolite crystals and the size of agglomerates may be thoseconventionally used.

The adsorbent compositions for use in the invention contain one or morenatural or synthetic molecular sieve zeolites capable of adsorbing watervapor from air without substantially selectively adsorbing nitrogen inpreference to oxygen. Molecular sieve zeolites are described in Breck,D. W. and Smith, J. V. "Molecular Sieves", Scientific American, 6(January 1959) and Breck, D. W. "Zeolite Molecular Sieves" (Wiley 1974).Molecular sieve zeolites can selectively adsorb molecules based ondifferences in the size, shape, and other properties, such as polarity,of molecules. Selective adsorption may be accomplished by selectingmolecular sieve zeolites for use in the composition based on theireffective pore diameter or based on their preferential affinity foradsorption of one molecular species over another.

Molecular sieve zeolites are crystalline aluminosilicates of group IAand group IIA elements such as sodium, potassium, magnesium, andcalcium. Structurally, zeolites are complex, crystalline inorganicpolymers based on an infinitely extending framework of AlO₄ and SiO₄tetrahedra linked to each other by the sharing of oxygen ions. Othermolecular sieve zeolites use phosphorous ions in place of some aluminumions. This framework structure contains channels or interconnected voidsthat may be occupied by the cations and water molecules. The water maybe removed reversibly, which leaves intact a crystalline host structurepermeated by micropores. The micropores of a particular type of zeolitehave a uniform pore size. The basic framework structure, or topology, ofthe zeolite determines the pore size and the void volume. The exchangecations, in terms of their specific location in the structure, theirpopulation or density, their charge and size, affect the molecular sievezeolite behavior and adsorption selectivity of the zeolite. By changingthe cation types and number, one can tailor or modify within certainlimits the selectivity of the zeolite. Molecular sieve zeolites adsorbselectively based on the size and shape differences between the crystalapertures and the adsorbate molecule. The adsorption selectivity of azeolite may also be based upon differences in the relative adsorptionaffinity between or among two or more coadsorbed gases or vapors. It ispossible to tailor the zeolite adsorption characteristics in terms ofsize selectivity or the selectivity caused by other interactions,including: cation exchange; cation removal or decationization; thepresorption of a very strongly held polar molecule, such as water;pore-closure effects, that is, effects which alter the size of theopenings to the crystal; and the introduction of various defects such asremoval of framework aluminum and changes in the silicon/aluminum ratio.

The basic types of commercially used molecular sieve zeolite adsorbentsare the 3A, 4A, 5A, and 13X sieves. The parent molecular sieve zeoliteof the Type A group is the 4A, which has sodium cations partiallyblocking the pores so that the effective pore opening is approximately4Å. Type 4A will adsorb molecules with critical diameters of less than0.4 nm. The type 3A sieve is a modification of the 4A in which most ofthe blocking sodium cations have been replaced by potassium. Thepotassium ion, being larger, blocks a greater portion of the poreopening and reduces its effective size. The 5A sieve is a modified 4A inwhich the blocking sodium cations have been replaced by calcium. Sinceeach calcium ion replaces two sodium ions, the number of cations isreduced. This causes most of the pore openings to be essentiallyunblocked and the resulting effective pore opening is nearly 5Å. Thecorresponding parent molecular sieve zeolite of the type X is also thesodium form. The crystal structure of type 13X results in an effectivepore opening of approximately 10Å diameter. In their current commercialform, types 4A, 5A, and 13X are not suitable for use in substantialamounts in adsorbent compositions of the invention.

A particularly preferred adsorbent composition consists essentially oftype 3A molecular sieve zeolite. Type 3A molecular sieve zeolite adsorbsessentially no oxygen or nitrogen. Thus no substantial oxygen enrichmentcan occur in thermally regenerated air driers which contain an adsorbentcomposition consisting of essentially type 3A molecular sieve zeolite.

Other adsorbents for use in the thermally regenerated air driers as partof an adsorbent composition also containing suitable molecular sievezeolites are activated aluminas and silica gels. Activated aluminas andsilica gels have disadvantages relative to molecular sieve zeolites suchas type 3A. Activated aluminas and silica gels readily adsorbhydrocarbons, some of which react to form coke deposits which act as afuel and as an adsorbent. Compressor lubricants are a common source ofhydrocarbons. Varying amounts of hydrocarbons present in ambient air areanother problem. Because only low molecular weight hydrocarbons desorbduring the reactivation steps, activated aluminas and silica gels canaccumulate sufficient hydrocarbons or coke deposits to cover nearly 100%of their surface area. The resulting hydrocarbon film or coke layer ofseveral hundred square meters per gram of adsorbent is an abundantquantity of accessible fuel. In comparison, hydrocarbons can enter onlythe macropores in the clay binder of 3A molecular sieve zeolite. Allhydrocarbons are too large to penetrate the 3 Angstrom apertures to theinterior cavities. The macropores of the clay binder of a typical 3Amolecular sieve zeolite bead or pellet comprise only about 1% of thetotal surface area of type 3A molecular sieve zeolite, amounting togenerally less than about 5 square meters per gram of adsorbent. Thususe of 3A molecular sieve zeolite effectively minimizes hydrocarbon andcoke deposits as a fuel. Coke deposits covering a significant fractionof the surface area of the myriad pores of activated aluminas and silicagels may also become an important secondary adsorbent. Specifically,coke deposits in activated aluminas and silica gels may behave likeactivated carbons which adsorb oxygen, sometimes preferentially relativeto nitrogen. Oxygen adsorbed on carbon has long been known to present agrave risk of fire and explosion, even at liquid air temperatures. Fireand explosion hazards involving coke deposits which adsorb oxygenpreferentially relative to nitrogen could be severe, primarily becauselarge quantities of molecular oxygen intimately contact the coke. Forthose coke deposits which absorb nitrogen preferentially relative tooxygen, the oxygen-enriched gas will present fire and explosion hazardsdownstream of the adsorber as well. For these reasons, it is notpreferred to employ activated aluminas or silica gels in the adsorbentcomposition of the invention.

The physical form of adsorbent composition, the quantity of adsorbentcomposition, and the other variables that determine the operation andefficiency of thermally regenerated air drying apparatus, chambers andprocesses of the invention, such as vessel size and configuration, cycletime, pressure, temperature and regeneration requirements, flowdirections, may be determined in the conventional way.

A Model To Illustrate The Problem

An isothermal equilibrium model was adapted to illustrate theperformance of thermally regenerated air driers which contain 13Xmolecular sieve zeolite. The nature of the oxygen enrichment problem isillustrated by predicting the nitrogen/oxygen breakthrough curves forthermally regenerated adsorbers which contain 13X molecular sievezeolite. This illustrates how nitrogen may adsorb preferentiallyrelative to oxygen in 13X molecular sieve zeolite during arepressurization or pressurization step, and that oxygen enrichedeffluent at high pressure may emerge when thermally regeneratedadsorbers are brought on line. This model illustrates the manner inwhich transient oxygen enrichment may occur in thermally regenerated airdriers and processes which experience an increase in pressure. Theactual operation of thermally regenerated air driers and processes maydeviate somewhat from the model for a variety of reasons.

FIG. 2 shows the predicted axial composition profiles at the end of arepressurization or pressurization step for thermally regeneratedadsorbers containing activated or freshly regenerated 13X molecularsieve zeolite. In all cases the adsorbers were assumed to containactivated or freshly regenerated molecular sieve zeolite and air at 1atm at the start of the repressurization or pressurization step. Air wasassumed to enter the adsorber during the isothermal repressurization orpressurization step until the total pressure reached 5.0, 7.5, or 10.0atm. The figure exhibits an oxygen mole fraction of roughly 60 percentat the closed (effluent) end of the adsorber.

FIG. 3 shows the predicted breakthrough curves that emerge from thethermally regenerated adsorbers during the subsequent adsorption step.The dimensionless time on the horizontal axis equals the number of bedvolumes of air that are assumed to have passed through the adsorber,based on an empty vessel. FIG. 3 indicates that the effluent compositionreturns to that of the air feed after approximately two bed volumes ofair have passed through the adsorber.

The presence of a high gaseous oxygen concentration widens theflammability range of hydrocarbons, markedly lowers the minimumautoignition temperature of hydrocarbons, and produces a much higherexplosion pressure and rate of explosion pressure rise compared with airat standard temperature and pressure ("STP"). In a gaseous mixture at agiven temperature, the partial pressure of oxygen rises with either anincrease in the oxygen mole fraction or an increase in the totalpressure. Both of these mechanisms operate during the repressurizationor pressurization of thermally regenerated air driers which containsubstantially 4A or 13X molecular sieve zeolites. Increasing the oxygenmole fraction above the 21% level found in air greatly expands theflammability range. Fires and explosions can be prevented by operatingoutside of the flammability range. The lower flammability limit ofhydrocarbons in air at STP is generally in the range of 1 to 5 molepercent, and it remains nearly independent of oxygen composition becauseit is fuel-limited. The upper flammability limit of hydrocarbons in airat STP varies from roughly 10 to 20 mole percent. As the oxygen molefraction increases above the 21% level of air, the upper flammabilitylimit of hydrocarbons increases. In the following equation, U_(c) is theupper flammability limit of hydrocarbon in mole percent due to oxygencomposition increase, and Y is the mole fraction of oxygen.

    ΔU.sub.c ≈70.0[log(Y)+0.679

Elevated pressures also widen the flammability range. While the lowerflammability limit of hydrocarbons decreases very slightly withincreasing pressure, the upper flammability limit of hydrocarbons due tototal pressure increases, UP, increases in proportion to the logarithmof the total pressure, P.

    ΔU.sub.p ≈20.6[log(P)-5.0]

The pressure and composition effects can be added together to find thetotal effect on the upper flammability limit of hydrocarbons.

FIG. 4 illustrates the increase in the upper flammability limit ofhydrocarbons as oxygen enriched effluent emerges from a thermallyregenerated adsorber containing 13X molecular sieve zeolite. Thedimensionless time of zero corresponds to the instant the adsorbervessel is brought on line. Here both the composition and total pressureeffects lead to an increase in the upper flammability limit ofapproximately 50 percent. At a dimensionless time of 4, oxygen-enrichedeffluent no longer emerges from the adsorber. Then the upperflammability limit increases by 15 to 20 percent because of the pressurecontribution only.

Increases in both the total pressure and the oxygen mole fractiondecrease the minimum autoignition temperature of hydrocarbons relativeto air at STP. The minimum autoignition temperature is a measure of thethermal energy required to initiate combustion; specifically, it is thelowest temperature at which the fuel spontaneously ignites. Table 1indicates that the minimum autoignition temperatures of kerosene andmineral oils fall to roughly 500K at a total air pressure of 10 atm.

The presence of iron oxides in rust catalyzes the

oxidation of oil and coke deposits, and lowers the minimum autoignitiontemperature of oils by roughly 50K. Other metal oxides may also promoteoxidation and lower the minimum autoignition temperature ofhydrocarbons.

Increasing the total pressure and oxygen mole fraction above that of airat 1 atm leads to a higher ultimate explosion pressure. The pressurecontribution follows from the ideal gas law, where the final pressure ina closed system increases in proportion to the initial pressure, theratio of final to initial moles of gas, and the ratio of final toinitial temperatures. ##EQU1## An increase in the oxygen mole fractionwill raise the ultimate explosion pressure if it allows more fuel toreact. Combustion of a volatile liquid increases the ultimate explosionpressure relative to a flammable gas since combustion of a volatileliquid produces a larger change in the ratio of moles of gas.

Additionally, increasing the total pressure and oxygen mole fractionabove that of air at 1 atm can produce a higher combustion rate, whichforces the explosion pressure to rise more rapidly and increases theseverity of an explosion. Although combustion kinetics can be verycomplex, the rate of reaction or combustion typically accelerates withincreasing fuel and oxygen partial pressures.

This model illustrates the nature of the problem solved by theinvention.

                  TABLE 1                                                         ______________________________________                                        Minimum Autoignition Temperatures                                             of Hydrocarbon Fuels in Degrees Kelvin                                        Pressure                                                                              Type of Hydrocarbon Fuel                                              (atm)   Kerosene (a)                                                                             Mineral Oils (b)                                                                            Mineral Oils (c)                             ______________________________________                                        0.25    866                                                                   0.50    737                                                                   1.0     502        623           568                                          10.0               523           535                                          100.0              473           502                                          ______________________________________                                         Notes:                                                                        a. J. M. Kuchta, S. Lambiris, and M. G. Zabetakis, "Flammability and          Autoignition of Hydrocarbon Fuels under Static and Dynamic Conditions", U     S. Bur. Mines Rep. Invest. 5992, 1962.                                        b. M. G. Zabetakis, G. S. Scott, and R. E. Kennedy, "Autoignition of          Lubricants at Elevated Pressures", U. S. Bur. Mines Rep. Invest. 6112,        1962.                                                                         c. C. S. McCoy and F. J. Hanly, "FireResistant Lubricants for Refinery Ai     Compressors", paper presented at the National Fuels and Lubricants Meetin     of the National Petroleum Refiners Association, September 11-12, 1975,        Houston, TX.                                                                  d. Normal hexane and longer normal paraffins have autoignition                temperatures of 480 to 500 K. in air at STP (F. T. Bodurtha, "Industrial      Explosion Prevention and Protection", McGrawHill, New York, 1980).       

What is claimed is:
 1. An apparatus for drying air comprising:(a) atleast one adsorbent chamber containing an adsorbent compositioncomprising one or more molecular sieve zeolites capable of adsorbingwater vapor from air, wherein such composition is substantially free ofmolecular sieve zeolites capable of substantially selectively adsorbingnitrogen in preference to oxygen, (b) means for providing an air stream,connecting such air stream to such chamber to adsorb water andincreasing the air pressure in such chamber at least once, (c) means forisolating such chamber from such air stream, and (d) means for heatingsuch adsorbent composition to substantially desorb water from suchadsorbent composition.
 2. An apparatus for drying air comprising:(a) atleast two adsorbent chambers containing an adsorbent compositioncomprising one or more molecular sieve zeolites capable of adsorbingwater vapor from air, wherein such composition is substantially free ofmolecular sieve zeolites capable of substantially selectively adsorbingnitrogen in preference to oxygen, (b) means for providing an air stream,connecting such air stream to such chamber to adsorb water, andincreasing the air pressure in such chamber at least once, (c) means forisolating such chambers from such air stream, (d) means for controllingsuch connecting and isolating means so that the air stream is directedthrough one such chamber to adsorb water while another such chamber isisolated from such air stream to desorb water, and (e) means for heatingsuch adsorbent composition to substantially desorb water from suchadsorbent composition.
 3. An apparatus of claim 1 or 2, wherein such oneor more molecular sieve zeolites capable of adsorbing water vapor fromair is type 3A molecular sieve zeolite.
 4. An apparatus of claim 1 or 2,wherein such one or more molecular sieve zeolites capable of adsorbingwater vapor from air is type 3A molecular sieve zeolite, and suchmolecular sieve zeolites capable of substantially selectively adsorbingnitrogen in preference to oxygen are types 4A and 13X molecular sievezeolites.
 5. An apparatus of claim 1 or 2, wherein such adsorbentcomposition comprises one or more molecular sieve zeolites capable ofadsorbing water vapor from air and one or more activated aluminas orsilica gels capable of adsorbing water vapor from air in amounts andtypes so that such composition is not capable of substantiallyselectively adsorbing nitrogen in preference to oxygen, and wherein suchcomposition is substantially free of molecular sieve zeolites capable ofsubstantially selectively adsorbing nitrogen in preference to oxygen. 6.An apparatus of claim 5 wherein such one or more molecular sievezeolites capable of adsorbing water vapor from air is type 3A molecularsieve zeolite.
 7. A method of air drying comprising:(a) adsorbing watervapor from an air stream by contacting such air stream with an adsorbentcomposition comprising one or more molecular sieve zeolites capable ofadsorbing water vapor from air, wherein such composition issubstantially free of molecular sieve zeolites capable of substantiallyselectively adsorbing nitrogen in preference to oxygen, and isolatingsuch adsorbent composition from such air stream, (b) increasing thepressure of air in contact with such adsorbent composition prior to orduring at least one such adsorbing step (a), and (c) heating suchadsorbent composition to desorb water.
 8. The method of claim 7, whereinsuch one or more molecular sieve zeolites capable of adsorbing watervapor from air is type 3A molecular sieve zeolite.
 9. The method ofclaim 7, wherein such one or more molecular sieve zeolite capable ofadsorbing water vapor from air is type 3A molecular sieve zeolite, andsuch molecular sieve zeolites capable of substantially selectivelyadsorbing nitrogen in preference to oxygen are types 4A and 13Xmolecular sieve zeolites.
 10. The method of claim 7, wherein suchadsorbent composition comprises one or more molecular sieve zeolitescapable of adsorbing water vapor from air and one or more activatedaluminas or silica gels capable of adsorbing water vapor in amounts andtypes so that such composition is not capable of substantiallyselectively adsorbing nitrogen in preference to oxygen, and wherein suchcomposition is substantially free of molecular sieve zeolites capable ofsubstantially selectively adsorbing nitrogen in preference to oxygen.11. The method of claim 10, wherein such one or more molecular sievezeolites capable of adsorbing water vapor from air is type 3A molecularsieve zeolite.